1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device including forming insulating gate films on a substrate.
2. Background Art
In a semiconductor device, a gate insulating film is generally formed between a gate electrode and an Si substrate to prevent tunneling current leakage The gate insulating film is generally formed of SiO2.
The gate insulating film has a big problem anticipated in future CMOS (complementary metal-oxide semiconductor) scaling. Specifically, as the size of semiconductor chips is reduced, the area of the channel region is also reduced. Therefore, in order to maintain the capacitance, the SiO2 film must also be thinned. However, if the thickness of the SiO2 gate insulating film is reduced to 2 nm or thinner, tunnel leak current occurs. Although the tunnel leak current can be tolerated in high-performance elements if it is 3 pA or below, it will become the same as off current at 1 nA, and will exceed the tolerable value at 50 μA.
On the other hand, according to ITRS (International Technology Roadmap for Semiconductor) 2001, the equivalent oxide thickness (EOT) will be required to be 1.5 nm or less in 2006 (70-nm generation).
Therefore, on and after this generation, since the tunnel leak current will exceed the tolerable value, the SiO2 film will not be able to be used as the gate insulating film. To cope with this problem, since the capacitance of an insulator is proportional to the dielectric constant, the tunneling leakage current can be suppressed if a metal oxide film (high-k material) is used as the gate insulating film to thicken the physical thickness of the gate insulating film.
However, when a metal oxide film is formed directly on an Si substrate from which a naturally formed surface oxide film has been removed by performing HF cleaning, an SiO2 film is grown in the interface between the Si substrate and the metal oxide film if allowed to stand in the atmosphere, and the defect density in the interface rises and the mobility lowers. Therefore, it has been proposed first to form an SiO2 film on an Si substrate, and overlay a metal oxide film on the SiO2 film to form a gate insulating film.